dt-bindings: riscv: add SBI PMU event mappings

The SBI PMU extension requires a firmware to be aware of the event to
counter/mhpmevent mappings supported by the hardware. OpenSBI may use
DeviceTree to describe the PMU mappings. This binding is currently
described in markdown in OpenSBI (since v1.0 in Dec 2021) & used by QEMU
since v7.2.0.

Import the binding for use while validating dtb dumps from QEMU and
upcoming hardware (eg JH7110 SoC) that will make use of the event
mapping.

Link: https://github.com/riscv-software-src/opensbi/blob/master/docs/pmu_support.md
Link: https://github.com/riscv-non-isa/riscv-sbi-doc/blob/master/riscv-sbi.adoc

 # Performance Monitoring Unit Extension
Co-developed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230113205435.122712-1-conor@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>